Suppressed P2-Z phase transition and Fe migration in the Na layer of an Fe/Mn-based layered oxide cathode for advanced sodium-ion batteries

Abstract

P2-type Fe/Mn-based-layered oxides have attracted extensive attention as potential candidates for sodium-ion batteries owing to their low cost, abundant reserves, and high theoretical capacity. However, when the cathode operates within the voltage range of 1.5–4.3 V, unexpected Fe-ion migration and P2-Z phase transition in this cathode cause rapid capacity degradation and remain insufficiently resolved. In this work, harmful P2-Z phase transition, as well as the migration of Fe ions, is completely mitigated. Specifically, we found that Ti-ion doping could dramatically improve the structural stability of the Fe/Mn-based cathode, hinder the formation of Fe⁴⁺O₆ in the sodium-ion layer, and promote the migration of Na⁺. The designed Na_0.67(Fe_0.5Mn_0.5)_0.495Ti_0.05O₂ (NFMT-2) cathode could exhibit a discharge capacity of 182.7 mA h g⁻¹ within the voltage range of 1.5–4.3 V (0.1C) and achieve a smaller cell volume change (1.26%) during cycling than the bare Fe/Mn-based cathode. This work presents a new approach to inhibit P2-Z unfavorable phase transition and Fe migration in the sodium layer of Fe/Mn-based layered oxides.